Why does carbon not form four bonds between carbon atoms?

When a single bond is formed between carbon atoms (or any atoms), energy is released. Changing the molecule's electron arrangement to make a double bond releases more energy, but not as much as when the first bond was made. read more

In the gas phase, it can. However, 8 electrons shared between two such small atoms leads to quite a bit of electron-electron repulsion, and less stability than spreading covalent bonds by bonding to more carbon atoms. read more

Carbon can not form 4 bonds with another carbon because of its orbitals which are some time hybridized. It shows the sp,sp2,sp3 hybirdization in various cases. In case of ethyne it shows sp hybirdization and two p unhybird orbitals forms two pi bonds with other atom and two hybrid orbitals forms sigma bonds one with carbon and other with hydrogen. read more

A quadruple bond requires one s bond, two p bonds and a d bond (between two unhybridized d orbitals). Valence-bond theory predicts two possible bonding states for C 2: a double bond with all electrons paired, and a triple bond with two unpaired electrons. read more